Base station for a telecommunication system

ABSTRACT

The invention concerns a unit for sending from a base station to terminals in a telecommunications system, said unit comprising at least two sending channels or groups of sending channels adapted to send on a plurality of adjacent carrier frequencies, said channels being able to send in spatial diversity mode so that the same information may be sent by the channels or groups of channels on the same carrier frequency or frequencies. In this unit the channels or groups of channels use at least one common carrier frequency for sending in spatial diversity mode and at least one of the channels or groups of channels comprises at least one different frequency for sending without spatial diversity or with a lower degree of spatial diversity.

The invention relates to a base station for a telecommunications system,in particular a system operating in accordance with the UMTS standard.

In a telecommunications system in which the terminals consist ofmobiles, the area over which the system extends is divided into cellsand a base station is provided in each cell. This is known in the art.Signals sent and received by a mobile pass in transit through the basestation assigned to the cell in which the mobile is located.

The base stations are generally the most costly items in this kind oftelecommunications system and the most costly component in a basestation is the power amplifier, whose cost increases in proportion tothe width of the band of frequencies that it must send. The wider theband of frequencies to be sent, the more difficult it becomes to ensurelinearity over the whole of the band combined with maximum efficiency.This problem is particularly acute under the UMTS standard, which usesthe wideband code division multiple access (WCDMA) technique in whichcodes are spread in frequency over a wide band, each modulated carrierhas a bandwidth of 5 MHz, and the set of bands that may be assignedcomprises twelve carriers and therefore spans 60 MHz.

Moreover, the cost of a base station may be further and significantlyincreased if it is necessary to provide for sending in spatial diversitymode, also under the UMTS standard. In the spatial diversity mode, thesame information is sent twice on the same carrier frequency and one ormore parameters are provided to differentiate the two radio signals sentand to prevent the undesirable effects of destructive combination of theradio signals. The two radio signals carrying the information may bedistinguished by a time offset or an orthogonal relationship between thesignals or by controlling their amplitude and phase differently, forexample.

Sending in spatial diversity mode reduces the probability oftransmission errors, especially under non-optimum propagationconditions. In particular, diversity is beneficial when the distancebetween the base station and the mobile is increasing, i.e. when themobile is approaching the boundary of the cell, and/or when thesignal-to-noise ratio or the signal-to-interference ratio necessary forcorrect reception is reduced, for example to send high bit rate data.

To enable sending in spatial diversity mode, two sending channels areprovided in each base station, with a wideband amplifier and an antennain each channel. Each of the two channels operates over the whole of theband assigned to the operator (or operators) using the channel, forexample three or four adjacent frequencies each occupying a bandwith of5 MHz.

Sending in spatial diversity mode therefore makes it obligatory toprovide two sending channels.

The invention stems from the realization that spatial diversity is notneeded all the time or for all mobiles, enabling the use of sendingresources to be optimized.

The base station of the invention comprises, in a manner that is knownin the art, at least two sending channels, each of which is able to sendon a plurality of carrier frequencies, and is characterized in that thesending channels have at least one common frequency to enable thespatial diversity mode to be used for sending on the common frequencyand at least one of the channels has at least one other frequency,different from the common frequency, for sending without using spatialdiversity.

The two sending channels preferably send simultaneously on the samenumber of frequencies and preferably have the same maximum sending powerand each is preferably configurable to send on frequencies that may beoffset relative to the frequencies of the other channel.

It may be shown that if two sending channels are provided, each of whichmay send on two adjacent carrier frequencies, one of which is common tothe two channels, the sending capacity is 10% to 20% greater than thecapacity obtained with two sending channels comprising three adjacentcommon frequencies. Moreover, the constraints imposed on each sendingchannel are reduced when using the invention (in the example given here,two frequencies are used instead of three).

The technology of amplifiers for UMTS base stations is such that theseamplifiers are able to process all twelve UMTS carriers but are able toprocess instantaneously only two, three or four adjacent carriers,corresponding to a bandwidth of 10 MHz to 20 MHz.

Thus the invention requires no change of technology, but insteadprogramming two (or more) sending channels differently, compared toprior art base stations. It suffices to offset the frequencies of onesending channel relative to the frequencies of the other sendingchannel.

The invention also applies when at least two groups of sending channelsare provided and all the channels in each group send in spatialdiversity mode on at least one frequency that is assigned only to thatgroup and at least one other sending frequency is assigned to all of thechannels of both groups to enable extensive spatial diversity, i.e.spatial diversity for all sending channels of both groups. In this case,the sending frequency assigned to the two groups enables a higher degreeof spatial diversity than the frequencies assigned to only one group.

The invention is generally concerned with a unit for sending from a basestation to terminals in a telecommunications system, said unitcomprising at least two sending channels or groups of sending channelsadapted to send on a plurality of adjacent carrier frequencies, saidchannels being able to send in spatial diversity mode so that the sameinformation may be sent by the channels or groups of channels on thesame carrier frequency or frequencies.

In this unit the channels or groups of channels use at least one commoncarrier frequency for sending in spatial diversity mode and at least oneof the channels or groups of channels comprises at least one differentfrequency for sending without spatial diversity or with a lower degreeof spatial diversity.

In one embodiment, each of the channels or groups of channels uses atleast one carrier frequency for sending without spatial diversity orwith a lower degree of spatial diversity.

At least one of the sending channels preferably comprises a poweramplifier able to send on a plurality of carrier frequencies and theactive frequencies of which may be selected by programming them.

One embodiment of the unit comprises means for modifying the sendingfrequency during a call or during a standby period and for each of theterminals in the area covered by the station, from a first frequencywith spatial diversity to a second frequency without spatial diversityor with a lower degree of spatial diversity than for the firstfrequency, and vice versa.

In this case, changing from a first frequency with spatial diversity toa second frequency without spatial diversity or with a lower degree ofspatial diversity, or vice versa, may be effected as a function of atleast one of the following parameters: the estimated position of theterminal relative to the base station, the bit rate of the call from thebase station to the terminal, the level or quality of the signalsreceived by the terminal, the power necessary for calling the terminal,and the class and grade of subscription of the terminal.

One embodiment of the unit comprises a device for assigning radioresources that is adapted to modify the sending frequency used for eachcall to a terminal.

The unit may further comprise two groups of channels where each groupcomprises at least one frequency used with spatial diversity only forthat group and at least one other frequency used with spatial diversityfor both groups.

The invention also consists in the application of a channel as definedhereinabove to a telecommunications system conforming to the UMTS, CDMA2000, or DOCOMO standard.

The invention further consists in the application of a channel asdefined hereinabove to a telecommunications system able to reassignfrequencies during a call and/or in a standby mode of the terminals.

Other features and advantages of the invention will become apparent fromthe description of certain embodiments thereof given with reference tothe appended drawings, in which:

FIG. 1 is a schematic of a telecommunications channel in which a basestation sends in spatial diversity mode;

FIGS. 2 a and 2 b are diagrams showing properties of sending channels ofa prior art base station;

FIGS. 3 a and 3 b are diagrams analogous to those of FIGS. 2 a and 2 bbut for a base station of the invention;

FIGS. 4 a and 4 b and 5 a and 5 b are diagrams analogous to those ofFIGS. 3 a and 3 b showing certain properties of the base station of theinvention; and

FIGS. 6 a, 6 b and 6 c are schematics showing variants of the invention.

In a UMTS telecommunications system the main components are the basestations, in particularly their sending part. Thus FIG. 1 represents thesending part 10 of a base station, which is referred to as a Node B inthe UMTS standard, and which comprises two sending channels 12 and 14,each of which comprises a multi-carrier amplifier 12 ₁ and 14 ₁ and anantenna 12 ₂ and 14 ₂, respectively. The base station 10 also comprisesa unit 16 controlled by a radio network controller (RNC) 18 andcontrolling the sending of signals and processing in the base band.

In the prior art, each channel 12 and 14 sends in the manner representedin FIGS. 2 a and 2 b, i.e. the two channels are identical and send atthe same power on three adjacent frequencies F₁, F₂, and F₃ (forexample) Thus the base station may send in diversity mode on each of thecarrier frequencies F₁, F₂, and F₃.

The FIGS. 3 a and 3 b diagrams represent one example of the propertiesof a base station of the invention. This base station is the same asthat represented in FIG. 1, i.e. it uses the same circuits (hardware),but is programmed differently. For example, the sending channel 12 sendson the same frequencies as in the prior art base station, i.e. on thefrequencies F₁, F₂, and F₃. However, the sending channel 14 isprogrammed to send on the frequencies F₂, F₃, and F₄, the frequency F₄being adjacent and higher than the frequency F₃. Under these conditions,the station may send in spatial diversity mode on the frequencies F₂ andF₃ and without using spatial diversity on the frequency F₁ on thechannel 12 and the frequency F₄ on the channel 14.

The device 18 assigns radio resources, i.e. frequencies, in a novelmanner. Accordingly, as represented in FIGS. 4 a and 4 b, if a mobile isclose to the base station at the beginning of a call, the device 18commands the base station to send on the frequency F₁ of the channel 12without using spatial diversity. If the mobile (not shown) subsequentlymoves away from the base station toward the boundary of thecorresponding cell, the device 18 modifies the frequency assigned,during the call, to enable sending in spatial diversity mode, i.e.sending on both channels 12 and 14. In this example, the frequency F₁ isdropped in favor of the frequency F₂ sent on the two channels 12 and 14.Changing from sending without diversity to sending with diversity isrepresented by the arrows f and f′ in FIGS. 4 a and 4 b. Under the UMTSstandard this is handled by the hard handover or interfrequency handoverfunction.

FIGS. 5 a and 5 b depict the operation of the same base station when themobile is ready to communicate. For example, if the mobile declaresitself in the cell on the frequency F₁ and the mobile is close to theboundary of the cell, then the device 18 commands the base station tosend from the base station to the mobile in spatial diversity mode. Thussending to this mobile will not use the frequency F₁ but the frequencyF₂ or F₃, which enables sending in spatial diversity mode.

In a similar way it is possible to modify the assignment of frequenciesbefore or during a call in order to change from a frequency with spatialdiversity to a frequency without spatial diversity. It is most importantto note that the device 18 may change the frequency assigned to sendingto each mobile independently, dynamically and in real time.

The examples referred to above allude primarily to the requirement forsending in spatial diversity mode when the mobile is moving away fromthe base station. However, changing between sending with and withoutspatial diversity may depend on other criteria. For example, a band offrequencies with spatial diversity is chosen if the information bit ratefor sending to the mobile is high and a sending frequency that does notsupport spatial diversity may be chosen when the bit rate is low. Thechoice of sending with or without spatial diversity may also depend onsuch criteria as:

-   -   quality of service criteria negotiated at the start of a call,        and    -   algorithms for providing and balancing resources of the RNC 18,        which may, for example, and depending on the requested range of        bit rates, decide to assign the carriers with spatial diversity        to all high bit rate mobiles and to assign the other carriers        without spatial diversity to “standard” mobiles.

Nor is the invention limited to the situation described hereinabove inwhich each base station may send on three adjacent frequencies, two ofwhich are common frequencies. Accordingly, as shown in FIG. 6 a, thechannel 12 sends on the frequencies F₁, F₂, and F₃ and the channel 14sends on the frequencies F₃, F₄, and F₅, i.e. only the frequency F₃ isused for sending in spatial diversity mode and the frequencies F₁, F₂,F₄, and F₅ are used for sending without spatial diversity.

In the example represented in FIG. 6 b, the channel 12 sends on thefrequencies F₁ and F₂ and the channel 14 sends on the frequencies F₂ andF₅. Note that in this case the number of carriers per amplifier is lowerand the power consumption per amplifier may therefore be reduced,because the linearization constraints are less severe. Moreover, asindicated hereinabove, capacity may be increased by approximately 10 to20% relative to a telecommunications channel in which each sendingchannel uses three adjacent carrier frequencies.

In the example shown in FIG. 6 c, the channel 12 sends on thefrequencies F₁, F₂, F₃, and F₄ and the channel 14 sends on thefrequencies F₃, F₄, F₅, and F₆. This configuration may be used when twooperators share the same infrastructure, in particular the same basestations. For example, the first operator uses the frequencies F₁, F₂,and F₃ and the second operator uses the frequencies F₄, F₅, and F₆.

Thus an amplifier technology with four carriers and only two amplifiersprovides a station serving two operators each with three carriers, eachoperator using spatial diversity on one of its carriers.

However, it is also possible for the operators to share their bands offrequencies.

The invention also applies when there are more than two sendingchannels, for example when using intelligent antennas. For example,eight channels may be provided, divided into two groups each of fourchannels, and the four channels in each group may use one (or more)spatial diversity frequencies for only these four channels and one (ormore) spatial diversity frequencies for all eight channels. In otherwords, the invention applies generally to the situation in which morethan one degree of spatial diversity is provided: a low degree ofspatial diversity for only some of the channels and a high degree ofspatial diversity for all the channels.

1. A unit for sending from a base station (10) to terminals in atelecommunications system, said unit comprising at least two sendingchannels or groups of sending channels (12, 14) adapted to send on aplurality of adjacent carrier frequencies, said channels being able tosend in spatial diversity mode so that the same information may be sentby the channels or groups of channels on the same carrier frequency orfrequencies, which unit is characterized in that the channels or groupsof channels use at least one common carrier frequency (F₂, F₃) forsending in spatial diversity mode and at least one of the channels orgroups of channels comprises at least one different frequency (F₁, F₄)for sending without spatial diversity or with a lower degree of spatialdiversity.
 2. A unit according to claim 1, characterized in that each ofthe channels or groups of channels uses at least one carrier frequencyfor sending without spatial diversity or with a lower degree of spatialdiversity.
 3. A unit according to claim 1, characterized in that atleast one of the sending channels comprises a power amplifier able tosend on a plurality of carrier frequencies and the active frequencies ofwhich may be selected by programming them.
 4. A unit according to claim1, characterized in that it comprises means for modifying the sendingfrequency during a call or during a standby period and for each of theterminals in the area covered by the station, from a first frequencywith spatial diversity to a second frequency without spatial diversityor with a lower degree of spatial diversity than for the firstfrequency, and vice-versa.
 5. A unit according to claim 4, characterizedin that changing from a first frequency with spatial diversity to asecond frequency without spatial diversity or with a lower degree ofspatial diversity, or vice versa, is effected as a function of at leastone of the following parameters: the estimated position of the terminalrelative to the base station, the bit rate of the call from the basestation to the terminal, the level or quality of reception of signals bythe terminal, the power necessary for calling the terminal, and theclass and type of subscription of the terminal.
 6. A unit according toclaim 4, characterized in that it comprises a device (18) for assigningradio resources adapted to modify the sending frequency used for eachcall to a terminal.
 7. A unit according to claim 1, characterized inthat it comprises two groups of channels and each group comprises atleast one frequency used with spatial diversity only for that group andat least one other frequency used with spatial diversity for bothgroups.
 8. Application of a channel according to claim 1 to atelecommunications system conforming to the UMTS, CDMA 2000, or DOCOMOstandard.
 9. Application of a channel according to claim 4 to atelecommunications system able to reassign frequencies during a calland/or in a standby mode of the terminals